Valerii Kotok

Nickel hydroxides (monohydroxides of several modifications and layered double hydroxides, based on nickel hydroxide) have high electrochemical, electrochromic, and electrocatalytic properties that allow them to be effectively used as an active substance in alkaline batteries, hybrid supercapacitors, electrochemical sensors, for electrocatalytic oxidation of organic substances and as an electrochromic device. Each application has its specific requirements for nickel hydroxides which must be defined. For an effective application, it is necessary to carry out a targeted construction of structure and composition, and selection of characteristics of nickel hydroxides and to develop targeted methods and conditions for nickel hydroxide synthesis. The structure, properties, and methods of preparation for use in various fields of nickel hydroxide application have been considered from the targeted ternary points of view: requirements definition, types and characteristics selection, and applying the synthesis method in industry. The 3‐level hierarchical complex of mechanisms for the formation of nickel hydroxides has been described. Also, the contribution of Ukrainian scientists to research in the field of nickel hydroxides is considered.

Nickel hydroxides are widely used as electrochemically active substances in alkaline batteries and hybrid supercapacitors; they can be used for electrocatalysis, in electrochemical sensors, and as pigments. Knowledge of the formation mechanism of nickel hydroxides is necessary for developing and optimizing targeted synthesis methods. The thermal effects of processes in the formation of nickel hydroxide from nitrate were studied by the calorimetry method. The mechanism of precipitate formation was investigated by the method of simultaneous potentiometric (with a glass universal electrode) and conductometric titration. The nickel content in the samples obtained at the determined NaOH/Ni2+ ratios was investigated by the chemical method of trilonometry after preliminary dissolution. Calorimetric investigations showed that the reaction of nickel nitrate with NaOH was exothermic with ΔНreaction=‒28328.5 J/mol. The exothermic nature of the NaOH dilution process was revealed with ΔНdilution =‒2454 J/mol. According to the results of potentiometric titration, the formation of a basic salt of the NiOHNO3 type was not detected. Analysis of the results of conductometric titration revealed a two-stage chemical mechanism for the formation of nickel hydroxide from nitrate. At the first stage, which had a high rate, due to the liquid-phase reaction of the nickel cation with the hydroxyl anion, a primary precipitate of the composition Ni(OH)1.87(NO3)0.13 was formed. In the second stage, as a result of a slow topochemical reaction of the primary precipitate with hydroxyl anions, nitrate ions were displaced from the precipitate to form nickel hydroxide. These data are confirmed by the analysis of precipitate obtained at NaOH/Ni2+ ratios of 1.87 and 2.2: the Ni content was 52.95 % and 55.63 %, corresponding to the formulas Ni(OH)1.87(NO3)0.13∙0.68H2O and Ni(OH)2∙0.71H2O. This clearly indicated that the primary precipitate was nitrate-doped α-Ni(OH)2 and the final precipitate corresponded to the α-modification of nickel hydroxide

Nickel foam is widely used as a current lead/current collector and as the base of nickel hydroxide electrodes for hybrid supercapacitors. An investigation of the influence of activation conditions for a commercial sample of nickel foam produced by Linyi Gelon LIB Co Ltd (China) was carried out using the method of impact nickel plating. The morphology of activated and non-activated nickel foam samples was investigated by scanning electron microscopy. Activated and non-activated nickel foam samples were investigated by methods of cyclic voltammetry and galvanostatic charge-discharge cycling in the supercapacitor mode. It was shown that upon activation at i=1 A/dm2 and τ=10 min, a thin layer of porous nickel with incomplete coverage was formed. Activation with impact nickel at i=7 A/dm2 and τ=3 min revealed the formation of a nickel coating with a highly developed surface, on which local cracks were found as a result of the accumulation of internal stresses. Activation with impact nickel at i=1 A/dm2 and τ=10 min led to the formation of a coating with a highly developed surface, with significant peeling of the coating. Cyclic voltammetry showed high efficiency of impact nickel activation at i=7 A/dm2, τ=3 min, and i=20 A/dm2, τ=5 min. The specific current of the cathode peak increased 6.06–6.44 times with respect to the non-activated sample. The investigation of the activated samples' electrochemical characteristics by the galvanostatic cycling method showed that impact nickel activation at i=1 A/dm2 and τ=10 min was insufficient. It was found that at a discharge up to E=0 V, the maximum specific capacitance of 0.731 F/cm2 was obtained for samples activated by impact nickel at i=7 A/dm2 and τ=3 min. The increase in specific capacitance compared to the non-activated sample was 4.49 times. At full discharge, the highest electrochemical activity was found for nickel foam samples activated by impact nickel at i=20 A/dm2 and τ=5 min. The specific capacitance was 0.505 mA∙h/cm2, and it increased 9.02 times

Pigments are the main components of cosmetics, which determine both toxicity and consumer color characteristics. Zn-Al layered double hydroxides intercalated with anionic food dyes are promising pigments. The best sources of dyes for intercalation are natural ones. The most promising are spices. Natural dyes from spices are often biologically active substances. The parameters of samples of Zn-Al (Zn:Al=3:1) hydroxides intercalated with natural food dyes, synthesized in the medium of aqueous tinctures of saffron and safflower, were studied. The crystal structure of the samples was studied by X-ray phase analysis; color characteristics were studied by spectroscopy and calculation of parameters in the CIE L*a*b system. The possibility of synthesizing Zn-Al colored layered double hydroxides intercalated with natural dyes in the medium of saffron and safflower tinctures was shown. X-ray phase analysis showed that both pigment samples were layered double hydroxides with the α-Zn(OH)2 structure. For the pigment intercalated with saffron dye, the phenomenon of partial decomposition of Zn-Al LDH to ZnO during synthesis was revealed. The color characteristics of the samples were studied. Zn-Al LDH pigment synthesized in a saffron tincture had a bright yellow color determined by intercalated saffron carotenoids (crocin and crocetin). It was suggested that safflower dye flavonoids were partially hydrolyzed (red ones - cartamine and cartamidine, and yellow - Safflor Yellow A), which led to the formation of a dark orange-brown color of the sample. The prospects of using Zn-Al LDH intercalated with saffron food dyes as a cosmetic pigment were shown

The cost of electrochromic “smart” windows ranges from 200–800 USD/m2. The high cost is determined by the small volume of production and the high cost of vacuum methods for applying device layers. The development of other methods could lead to lower costs and further mass use of “smart” windows. The paper presents the work on improving the electrochemical method of deposition of electrochromic films based on Ni(OH)2. Composite electrochromic films were deposited from Ni(NO3)2 solutions with the addition of water-soluble polymers – polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and mixtures thereof. Two PVA grades and one PVP grade were used in the experiments. As a result, six films were obtained from solutions of different compositions and concentrations of polymers. An investigation of the optical properties of the films in the initial state showed that there is no intense absorption of light in the entire visible spectrum and the transmission corresponds to a glass substrate. Thus, the formed electrochromic films will not produce tints and visible light absorption in the finished electrochromic element. The electrochemical and electrochromic characteristics of the films differed significantly depending on the amount and specific polymers used. The film obtained from a solution containing PVA grade 24-99 was the most electrochemically active, while the best electrochromic characteristics were in the film deposited from a solution of a mixture of PVP grade 24-99 and PVP. Moreover, the films obtained in solutions containing separately PVA grade 24-99 and PVP had worse electrochromic characteristics, and the electrochemical characteristics were the same or worse relative to the film obtained in a mixture of polymers. The results of the method of electrochemical impedance spectroscopy showed compliance with the found electrochromic and electrochemical characteristics of the formed films.